1. Field of the Invention
The present invention generally relates to tilt correction for an object lens relative to an information recording medium in an optical disk recorder/player, and more particularly, to a method and apparatus for correcting tilt relative to an information recording medium through superposition of signals.
2. Description of Related Art
With an optical disk apparatus (i.e., an optical disk recorder/player), a laser beam is guided onto the recording side of an information recording medium, such as an optical disk, to record information. The recorded information is reproduced based on the light reflected from the recording side. In general, the optical disk apparatus is provided with an optical pickup device, which emits a laser beam to form a light spot on the recording side of the information recording medium, as well as receiving light reflected from the recording side.
Generally, an optical pickup device has an optical system including an object lens, which guides a laser beam emitted from a light source onto the recording side of the optical recording medium and guides return light flux reflected from the recording side to a prescribed light-receiving position. The optical pickup device also has a light receiving element arranged at the light-receiving position to receive the return light flux. The light receiving element outputs not only information reproduced from the data recorded in the recording side of the medium, but also signals containing servo information required for positioning of the object lens and the optical pickup device itself.
In recent years and continuing, the recording density in information recording media is increasing in response to demand for increased recording capacities of information recording media. In order to increase the recording density, the diameter of the light spot formed on the recording plane has to be decreased. To achieve this, use of an object lens with a large numerical aperture is becoming popular. However, as the numerical aperture increases, influence of wavefront aberration due to offset of the optical axis of the object lens from a line perpendicular to the recording plane also increases. The offset of the optical axis of the object lens is referred to as “tilt”, which is likely to cause deformation of the light spot, as well as degradation of signals output from the light receiving element and containing reproduced information and servo information.
There are two types of tilt correction, namely, a method for inclining the information recording medium relative to a fixed object lens, and a method for inclining the object lens relative to the information recording medium. The recent trend is employing the latter method due to demand for reducing the size of the tilt correction mechanism and for increasing the tilt correction speed. To adjust the inclination of the object lens, a tilt control mechanism is known, which is designed to bring the guide shaft attached to a fixed part of the optical pickup device into an opening formed in the movable part having the object lens and to rotate the movable part of the optical pickup device about the guide shaft against a frictional force between the movable part and the guide shaft.
For example, JPA 2000-260042 discloses a control apparatus for driving a carriage of the optical pickup device against a force of static friction.
However, driving the movable part of the optical pickup device against the frictional force, as in the above-described tilt correction technique using a tilt control mechanism, has a problem in that smooth motion of the movable part is hindered by the frictional force and the movable part cannot follow up the driving signal used to correct the inclination of the object lens. Consequently, the correction accuracy is likely to be degraded.